1. Field of the Invention
This invention relates to a nitride semiconductor substrate, a production method therefor, and a nitride semiconductor device. In particular, it relates to a group III nitride semiconductor single crystal substrate, a production method therefor, and a nitride semiconductor device using these.
2. Description of the Related Art
For blue light emitting diode (LED) or laser diode (LD) materials, nitride semiconductors, for example, group III nitride semiconductors such as gallium nitrides (GaN), indium gallium nitrides (InGaN), aluminum gallium nitrides (AlGaN), and the like, are in the limelight. Further, for electronic device elements, the application and development of group III nitride semiconductors have begun because of their good heat resistance or environment resistance.
In order for these devices to have enhanced performance, it is important to lessen crystal defects (principally, dislocations) in an epitaxial layer (a semiconductor layer epitaxially grown over a substrate) of the devices. Originally used for the above substrate in the nitride semiconductor device fabrication is a hetero-substrate such as sapphire or the like. The hetero-substrate causes high density dislocations due to its lattice constant being significantly different from the lattice constant of the epitaxial layer. To avoid this to produce the high quality epitaxial crystal (semiconductor crystal epitaxially grown over the substrate), effectively used for the above substrate is a high quality nitride semiconductor single crystal substrate, which is highly lattice matched to the epitaxial layer.
Of such nitride semiconductors, the GaN single crystal is not easy to grow because its melt is produced only at an ultra high pressure, but attempts to grow the GaN single crystal have been made with various methods such as an ultra high temperature and high pressure method, a flux method, an HVPE (halide vapor phase epitaxy) method, and the like. Of these, the HVPE (halide vapor phase epitaxy) method is the most developed GaN substrate production method. Specifically, HVPE produced GaN freestanding substrates (substrates having a strength not only being capable of holding its shape but causing no inconvenience in handling) have already been available commercially mainly for use in optical disc LDs, and are greatly expected for future use in super luminosity LEDs, as well as LDs.